Application Languages in Software Production

PRL5 is an application-oriented language used to maintain the integrity of databases in the AT&T 5ESSTM telecommunications switch. PRL5 is unusual in that it was explicitly designed to eliminate a number of different coding and inspection steps rather than simply to improve individual productivity. Because PRL5 replaced an earlier high-level language named PRL, which in turn replaced a combination of English and C on the same project, it is possible to trace the effect of several fundamentally different languages on this single project. The linguistic evolution has been away from languages describing computation toward a "declarative" high-level language that has been deliberately restricted to accommodate the requirements of certain analyses. Algorithms for checking database constraints are no longer specified by human developers; instead, code is generated from static representations of the constraints themselves. These constraint descriptions can be used in more than one way, whereas a program to check constraints is useful only for performing that particular computation. In effect, PRL5 allows the re-use of project information at a high level, before it has been specialized into particular implementations. The effects of this re-use on quality, interval, and cost are tangible. A key lesson is that application-oriented languages should not be designed to describe computation, they should be designed to express useful facts from which one or more computations can be derived.